Magnetic structure for focusing of linear beams

ABSTRACT

A high convergence magnetically focused linear beam tube includes a pair of magnetic pole piece structures at opposite ends of the interaction circuit. At least one of the magnetic pole piece structures includes a centrally apertured transverse wall and an axial tubular projection extending away from the transverse wall for shaping the magnetic field externally of the region of the interaction circuit. The diameter of the central aperture is predominantly determinative of the gradient of the convergent axial magnetic field in the high field region, i.e., greater than 50 percent of the maximum axial magnetic field especially in the region of the beam near the beam minimum. The beam aperture is surrounded by a non-magnetic gap in the pole piece structure for independently controlling a certain amount of leakage of magnetic field through the pole piece structure to predominantly determine the magnitude of the axial magnetic field in the low field region, i.e., less than 10 percent of the maximum axial magnetic field intensity externally of the interaction circuit. Controlling the gap width is useful in the case of permanent magnet focusing for moving the point of magnetic field reversal away from the interaction circuit at either the collector or cathode end of the tube to obtain a desired shape of the magnetic flux divergence or convergence.

Unite States atent [191 Nelson et al.

1 1 MAGNETIC STRUCTURE FOR FOCUSING OF LINEAR BEAMS [75] Inventors: Richard B. Nelson; Erling L. Lien,

both of Los Altos; George V. Miram, Redwood City, all of Calif.

[52] US. Cl 315/35, 313/84, 315/535 [51] Int. Cl. HOlj 25/34 [58] Field Of Search 315/534, 5.35, 3.5; 313/84 [56] References Cited UNITED STATES PATENTS 2,608,668 8/1952 Hines 313/84 X 2,701,321 2/1955 Rich 313/84 2.774,006 12/1956 Field et a] 313/84 X 2,905,847 9/1959 Klein 61 a1. 313/84 X 3,155.866 11/1964 P0018 v 313/84 X 3,522,469 8/1970 Miram 315/535 X Primary Examiner-James W. Lawrence Assistant EraminerSaxfield Chatmon, Jr.

Attorney, Agent, or FirmDavid Roy Pressman; R. K. Stoddard; H. E. Aine Aug. 27, 1974 157] ABSTRACT A high convergence magnetically focused linear beam tube includes a pair of magnetic pole piece structures at opposite ends of the interaction circuit. At least one of the magnetic pole piece structures includes a centrally apertured transverse wall and an axial tubular projection extending away from the transverse wall for shaping the magnetic field externally of the region of the interaction circuit. The diameter of the central aperture is predominantly determinative of the gradient of the convergent axial magnetic field in the high field region, i.e., greater than 50 percent of the maximum axial magnetic field especially in the region of the beam near the beam minimum. The beam aperture is surrounded by a non-magnetic gap in the pole piece structure for independently controlling a certain amount of leakage of magnetic field through the pole piece structure to predominantly determine the magnitude of the axial magnetic field in the low field region, i.e., less than 10 percent of the maximum axial magnetic field intensity externally of the interaction circuit. Controlling the gap width is useful in the case of permanent magnet focusing for moving the point of magnetic field reversal away from the interaction circuit at either the collector or cathode end of the tube to obtain a desired shape of the magnetic flux divergence or convergence.

8 Claims, 10 Drawing Figures MAGNETIC STRUCTURE FOR FOCUSING OF LINEAR BEAMS BACKGROUND OF THE INVENTION The present invention relates in general to magnetic structure for focusing of linear beams and more particularly to an improved pole piece configuration for highly convergent magnetically focused electron beams.

DESCRIPTION OF THE PRIOR ART Heretofore, microwave linear beam tubes, such as klystrons and traveling wave tubes, have employed pole piece structures at opposite ends of the beam path shaped so as to produce a convergent or divergent shaped bundle of magnetic flux lines at opposite ends of the tube externally of the interaction circuit to magnetically confine or focus the'electron stream entering and exiting the interaction circuit.

In the case of the pole piece structure at the gun end of the tube, this pole piece generally has included a transversely directed wall of a magnetic permeable material such as soft iron having a beam aperture therein through which the beam is drawn into the magnetic focusing field between pole piece structures at opposite ends of the tube.

The minimum diameter of the beam hole is generally dictated by the requirement that the flux lines of the magnetic beam focus field conform to the electron trajectories in the region of the beam minimum. The axial location of the transverse wall of the pole piece is prescribed by the requirement that the beam minimum, as dictated by the electrostatic field of the electron gun, occur at a point of approximately 80 to 95 percent of the maximum beam focus magnetic field intensity. The axial magnetic field intensity along the axis of the beam hole in the pole piece is related to the beam hole size by the relation that at one hole radius on either side of the beam hole the axial magnetic field is within 7 percent of zero on the low field side and within 7 percent of the maximum field value on the high field side.

In addition, the pole piece structure at the gun end of the tube typically includes a tubular projection extending around the cathode emitter so as to cause the beam focus magnetic field lines passing through the beam aperture to converge along paths, at the periphery of the beam which generally coincide with the shape of the convergent electrostatic beam focusing lines at the beam edge so as to produce a magnetically confined laminar flow electron beam. Examples of such prior art magnetic beam focusing structures are disclosed in US. Pat. Nos. 3,522,469 issued Aug. 4, 1970 and 3,331,984 issued July 18, I967, both assigned to the same assignee as the present invention.

Such prior art magnetic beam focusing structures employing solenoid energization have typically been adequate for focusing electron beams having an area convergence less than 10 to 1. When the area convergence is less than 10 to l the beam aperture in the anode pole piece can be relatively large and the point of axial magnetic field reversal, when using permanent magnet excitation of the pole structures, occurs outside of the region between cathode and anode because the cathode is relatively close to the anode.

However, in the case of a highly convergent electron gun, i.e., guns having an area convergence greater than 50, the beam aperture in the anode or gun pole piece structure is generally smaller in diameter than the cathode emitter and in the case of permanent magnet excitation the point of external axial magnetic field reversal moves in much closer to the transverse plane of the beam aperture. In a typical example of a high perveance X-band tube, having a beam area convergence in the gun of greater than 50, the point of field reversal occurs between the cathode emitter and the beam aperture in the gun pole piece structure. In such a case, the magnetic field lines in the region of the gun cannot conform to the electrostatic field lines and so magnetically confined flow beam focusing cannot be achieved with the prior art beam focusing structures.

SUMMARY OF THE PRESENT INVENTION The principal object of the present invention is the provision of an improved magnetic structure for focusing of a linear beam.

In one feature of the present invention, the centrally apertured magnetic pole piece of the beam focus structure includes a magnetic gap of non-magnetically permeable material surrounding the beam aperture so as to increase the amount of leakage flux through the pole piece structure to predominately control the magnitude of the axial magnetic field in the low field region outside of the pole piece structure independently of the magnitude of the axial magnetic field in the high field region.

In another feature of the present invention, the magnetic pole piece structure at the gun end of the tube is energized by a permanent magnet and includes a centrally apertured transverse wall portion and an axially directed tubular projection surrounding the cathode emitter of the gun. An annular non-magnetically permeable gap surrounds the beam aperture in the transverse wall of the pole piece, such gap being dimensioned so as to increase the leakage of magnetic flux through the pole piece structure and to move the point of axial magnetic field reversal outside of the region between the cathode emitter and anode.

Other features and advantages of the present invention will become apparent upon a perusal of the following specification taken in connection with the accompanying drawings wherein:

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a side elevational view, partly in section, of an electron beam tube incorporating features of the present invention,

FIG. 2 is an enlarged sectional view of a portion of the structure of FIG. 1 delineated by line 22,

FIG. 3 is a plot of normalized axial magnetic field intensity vs. distance within that region of the gun of FIG. 2 delineated by line 33,

FIG. 4 is a simplified line diagram of the pole piece structure of FIG. 2 showing a beam focus flux line passing through the periphery of the cathode emitter,

FIG. 5 is a schematic simplified line diagram depicting the magnetic flux and electrostatic flux lines in the region of the electron gun,

FIG. 6 is a schematic longitudinal sectional view of a permanent magnet beam focus structure of the prior art depicting the field reversal in the region of the electron gun,

FIG. 7 is an enlarged detail view of a portion of the structure of FIG. 6 showing correction of the beam focus magnetic field in the region of the gun by the-provision of a magnetic gap in the pole piece structure, and

FIGS. 8-l0 are longitudinal sectional views of alternative pole piece structures incorporating features of the present invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS Referring now to FIG. 1, there is shown a typical X- band microwave linear beam tube 11 incorporating features of the present invention. The microwave tube 11 includes an electron gun assembly 12 disposed at one end of the tube for forming and projecting a beam of electrons 13 over an elongated beam path to beam collector structure 14 disposed at the terminal end of the tube for collecting and dissipating the energy of the beam. A microwave interaction circuit 15 is disposed between the gun 12 and the collector 14 and along the beam path 13 for electromagnetic interaction with the beam to produce output microwave energy which is extracted from the circuit 15 via a conventional coupling means 16 such as an iris and output wave guide.

In a typical example, the interaction circuit 15 comprises a plurality of axially spaced cavity resonators for successive electromagnetic interaction with the electron beam in the conventional manner of a klystron tube. In the case of a klystron amplifier, input microwave energy to be amplified is applied to the upstream resonator of the circuit 15 by a conventional input coupling means 17 such as a waveguide and coupling iris.

Beam 13 is magnetically focused through the interaction circuit 15 by means of an axially directed magnetic field produced between a pair of pole piece structures 18 and 19 disposed at the gun end and collector end, respectively, of the interaction circuit 15. In a preferred embodiment, the pole pieces 18 and 19 are magnetically energized by annular radially polarized permanent magnets 21 and 22 disposed surrounding the pole piece structures 18 and 19, respectively. A cylindrical magnetic yoke structure 23, as of soft iron, surrounds the microwave interaction circuit 15 and interconnects the outer poles of the respective magnets 21 and 22. The yoke 23 is apertured in alignment with the waveguide coupling assembly 16 and 17. Such a permanent magnet beam focus structure is disclosed and claimed in copending US. application Ser. No. 291,117 filed Sept. 21, 1972 and assigned to the same assignee as the present invention.

Referring now to FIG. 2, there is shown in greater detail the electron gun assembly 12 and associated pole piece structure 18. The electron gun assembly includes a thermionic cathode emitter member 25 having a spherically concave cathode emitting surface 26 axially aligned with the center line 27 of the beam path 13. The electron gun assembly 12 includes a centrally apertured anode electrode 28 said anode having an outwardly flared beam entrance aperture 29 coaxially aligned with the center line of the beam path 27. An annular beam focus electrode 31 operated at cathode potential is disposed between the cathode 25 and the anode 28 in surrounding relation to the beam path 13 to aid in obtaining the proper shape for the electrostatic beam focus lines in the region between the cathode and the anode.

A multiapertured spherically concave control grid 32 is closely spaced to the emitting surface 26 of the cathode for controlling the beam current. A multiapertured shadow grid 33 operated at cathode potential is interposed between the control grid 32 and the cathode emitting surface 26 with the webs of the grid being in registration with the webs of the control grid 32. The cathode emitting surface 26 is also preferably dimpled with spherically concave emitting surfaces, each concave emitting surface being in axial alignment with the corresponding aligned apertures in the shadow grid 33 and control grid 32.

The anode focus electrode 31 and cathode 25 are shaped and dimensioned relative to each other to provide a relatively high degree of area convergence of the beam due to the electrostatic focusing forces on the beam. For example, in a preferred embodiment of the present invention the area convergence of the beam is greater than 50 and in a typical example is 65. Area convergence is the ratio of the cross-sectional area of the beam at the beam maximum, i.e., in the transverse plane at the periphery or outer lip of the cathode emitter surface 26 to the cross-sectional area of the beam at the beam minimum. The point of beam minimum diameter is determined by the electrostatic focusing forces, such beam minimum occurring at a point just inside of the constricted neck portion 35 of the anode 28, namely, at transverse plane 36.

The magnetic pole piece structure 18 includes a hollow cylindrical portion 38 coaxially disposed of and surrounding the cathode emitter 25 and a transverse wall portion 39. The transverse wall portion 39 is centrally apertured at 41 in coaxial alignment with the center line of the beam 27. The inside diameter of aperture 41 is chosen such that the magnitude of the beam focuse magnetic field on the axis 27 of the beam path 13 at the beam minimum 36 has a value falling within the range of to percent of the maximum axial magnetic field produced along the beam axis 27 between pole pieces 18 and 19, respectively. In a typical example of an X-band klystron, this maximum value of magnetic field has a value of approximately 2,000 gauss.

Cylindrical or tubular projection portion 38 of the pole structure 18 is dimensioned and shaped relative to the transverse wall portion 39 to cause the magnetic flux lines passing through the central aperture 41 in the pole piece 18 to coincide with the shape of the electrostatic field lines at the outer periphery or envelope of the beam 13 in the region between the cathode emitter surface 26 and the constricted neck portion of the anode.

An annular non-magnetic gap 43 is provided in the transverse wall portion 39 of the pole piece structure 18. The non-magnetic gap 43 coaxially surrounds the axis 27 of the beam 13 and serves to provide a controlled increase in magnetic field leakage through the pole piece structure 18 in such a manner as to move the axial point of field reversal, in the case of a permanent magnet energized pole structure 18 and 19, away from the transverse wall 39 of the pole piece structure 18 and more particularly to a point outside of the cathode emitting surface 26 in order to obtain the aforedescribed proper shape for the beam focus magnetic field in the region of the gun. The non-magnetic gap 43, in a preferred embodiment, is filled with a thermally conductive non-magnetic material such as copper tofacilitate thermal conduction from the anode 28 through the pole piece structure to the surrounds.

Referring now to FIG. 3, there is shown a plot of normalized axial magnetic field intensity vs. distance within the gun with the outline of certain of the electrodes shown superimposed on the plot. The plot of FIG. 3 shows the function of the annular non-magnetic gap 43. More particularly, solid curve 45 and its times expanded scale counterpart 46 correspond to transverse wall 39 without the provision of the annular magnetic gap 43. As can be seen, particularly by curve 46, a field reversal occurs at point 47. Such a field reversal between the cathode and the anode makes it impossible to obtain proper magnetically confined flow beam focusing in the region of the gun.

In the electron gun 12, the spacings of the electrodes and sizes of the electrodes are determined by the area convergence of the beam. For an area convergence of 65, as indicated in FIG. 3, a solid transverse magnetic wall 39 does not permit confined flow magnetic focusing. The inside diameter of the hole 41 in the pole piece 18 is chosen to produce an axial magnetic field intensity at the point of the beam minimum 36 falling within the range of 80 to 95 percent of the maximum magnetic field intensity. Opening the size of hole 41 to allow increased magnetic field leakage within the gun region to move the point of field reversal away from the structure would decrease the magnetic field intensity at the beam minimum. Thus, the size of the beam aperture 41 in the pole piece structure is dictated by the requirements of the magnetic field at the beam minimum such beam minimum being determined by the electrostatic beam optics of the gun. The diameter of the beam hole 41 predominately controls the axial gradient of the axial magnetic field within the vicinity of the beam hole, i.e., within one hole radius on either side of the transverse plane passing through the point where the axial magnetic field on the axis of the beam hole is of a value of 50 percent of B The provision of the non-magnetic gap 43 results in curve 48 and its expanded scale counterpart 49. From curve 49 it is seen that the point of field reversal is moved out of the region between the cathode and anode and provides a finite axial magnetic field intensity at the cathode surface 26 of approximately 1.5 percent. Thus, the non-magnetic gap 43 allows control over the magnitude of the axial magnetic field in the low field region, i.e., field intensities less than 10 percent of B such control being independent of the slope of the field in the high field region, i.e., greater than 10 percent of B The magnetic field intensity at the cathode B for magnetically confined flow beam focusing is deter mined from the following relation:

c MAX C where B is the maximum axial magnetic field intensity between the pole pieces 18 and 19 on the axis of the beam 27; a is the percent magnitude of the axial magnetic field at the point of the beam minimum 36 relative to B and A is the area convergence of the gun. For an area convergence of 65 and a B of 2,000 gauss with 90 percent of B attained at the point of beam minimum, the axial magnetic field B, at the cathode is approximately 1.5 percent of B FIG. 4 shows by curve 51 the outer envelope of the beam focus flux tube for magnetically confined, laminar flow of the electron beam at the outer periphery of the beam in the region of the gun. More particularly, curve 51 represents that the flux lines at the outer surface of the beam are coincident with the shape of the electrostatic field lines at the beam edge.

Referring now to FIGS. 5-7, there is shown the mode of operation of the non-magnetic gap 43. More particularly, FIG. 5 shows that in the absence of the nonmagnetic gap 43 the magnetic flux lines at the periphery of the beam in the region of the gun, as shown by dotted line 52, converge too rapidly if the pole piece beam aperture is chosen to make the flux lines properly match the electron trajectories in the vicinity of the beam minimum. If the pole piece is proportioned as shown by the solid lines, to produce the proper flux at the cathode, as indicated by the dashed lines, it is seen that the magnetic flux lines are then a poor match for the electron trajectories in the area of the beam minimum. A purpose of the present invention is to resolve these incompatible requirements. FIG. 6 shows the flux lines of a permanent beam focus magnet, and particularly in the region of the cathode 26. The field reversal is depicted and it can be seen that the leakage flux passes through the surface of the cathode emitter in the opposite direction to the direction of the main field, thereby producing a field reversal between the cathode and the anode.

Referring now to FIG. 7 it is shown how the nonmagnetic (low permeability) gap 43 increases the reluctance of the pole piecestructure 18 to the main field component, thereby causing more of the main field to pass through the pole piece structure and to move the point of field reversal to the opposite side of the cathode. The corrected field is shown by dotted line 53.

In a typical example of the non-magnetic gap of FIG. 2, the gap 43 has a radial thickness of approximately 0.020 inches and an axial extent of 0.150 inches. The axial offset shoulder at 55 between the inner portion 56 and the outer portion 57 of the pole structure 39 is for the purpose of smoothing the shape of the magnetic flux tube 51 in the region between the cathode and anode.

Referring now to FIGS. 8-9 there are shown a number of alternative pole piece embodiments to that previously described with regard to-FIG. 2. In the embodiments of FIGS. 8-10 the non-magnetic gap 43 is shown as combinations of axial and radial gap portion or, in the case of FIG. 9, purely a radial gap.

Although the non-magnetic gap for increased flux leakage has thus far been described as employed for shaping the field in the region of the cathode emitter to move the point of field reversal outwardly of the pole structure, it may also be utilized to advantage in the collector pole piece structure 19 for moving the point of axial magnetic field reversal further toward the end or completely outside of the end of the collector structure 14. This will permit a more uniform expansion of the beam in the collector region to obtain a more uniform distribution of the energy of the beam over the interior surfaces of the collector. More particularly, if the field reversal is in too close to the collector pole piece structure 19 the beam in passing into the collector is given too high a rotational energy resulting-in reflection of low velocity electrons back along the beam path during RF operation of the beam tube.

What is claimed is:

1. In a linear beam tube:

electron gun means for forming and projecting a beam of electrons along trajectories extending over an elongated beam path;

collector means at the terminal end of the beam path for collecting and dissipating the energy of the collected electrons;

electrical circuit means disposed along the beam path intermediate said gun means and said collector means in energy exchanging relation with the beam to produce output electromagnetic wave energy;

means for magnetically focusing the beam over said beam path between said electron gun means and said collector means, said magnetic beam focusing means including, first and second pole piece struc tures disposed along said beam path in axially spaced relation relative to said beam path, each of said pole piece structures being made of a magnetically permeable material, magnet means for energizing said pole piece structures for producing an axially directed beam focus magnetic field composed of flux lines within the beam path, said first one of said pole piece structure having an aperture through which the beam passes, said aperture having a diameter such as to predominately determine the gradient of the axial field in the field region within one hole radius on either side of the transverse plane corresponding to an axial magnetic field intensity of 50 percent of the maximum axial magnetic field intensity between said pole pieces, said aperture diameter being so selected as to cause the flux lines of said magnetic field to conform to said electron trajectories within said region within one hole radius, said first pole piece structure also having a magnetic gap portion of lower magnetic permeability than that of the adjacent portion of said first pole piece structure, said gap being disposed surrounding said beam aperture in said first pole piece structure to allow an increase in the magnetic flux leakage through said first pole piece structure and being dimensioned so as to predominately determine the magnitude of the axial mag netic field intensity in the region of field external to said first and second pole pieces and in the region where the magnitude of the axial magnetic field is less than 10 percent of said maximum magnetic field intensity.

2. The apparatus of claim 1 wherein said nonmagnetically permeable gap is filled with a nonmagnetic metal material.

3. The apparatus of claim 1 wherein said electron gun includes, a thermionic cathode emitter having a concave cathode emitting surface facing said first pole piece structure, and a centrally apertured anode electrode spaced from said cathode for drawing a convergent flow of electrons from said cathode into said beam passing through said central aperture in said anode;

said first pole piece structure being disposed facing said cathode and being shaped to produce a convergent bundle of magnetic flux lines passing through the peripheral area of the concave emitting surface of said cathode which substantially coincides with the convergent shape of the electrostatic field lines in the same region between said cathode and anode to produce a magnetically confined substantially laminar convergent flow of electrons from said cathode through said first pole piece structure.

4. The apparatus of claim 3 wherein said gun is dimensioned to produce an area convergence of said beam in excess of 50.

5. The apparatus of claim 1 wherein that portion of said first pole piece structure surrounded by said gap portion is connected electrically to said anode for operating at anode potential.

6. The apparatus of claim 4 wherein said first pole piece structure includes an axial tubular projection of larger diameter than said beam aperture and being disposed surrounding said concave cathode emitting surface for shaping the beam focus magnetic field lines in the region of said concave cathode emitter.

7. The apparatus of claim 6 wherein said first magnetic pole piece structure including the non-magnetic gap portion thereof is shaped to cause the magnetic field in the region of said electron gun to satisfy the relation t MAX C where A is the area convergence of the electron beam; B is the maximum value of axial magnetic field in the beam path intermediate said first and second pole piece structures; a is the ratio of axial beam focus magnetic field in the beam path at the point of minimum beam diameter to B and B is the value of axial magnetic field at the center of the concave cathode emitting surface.

8. The apparatus of claim 1 wherein said magnet means for energizing said pole piece structures is a permanent magnet means. 

1. In a linear beam tube: electron gun means for forming and projecting a beam of electrons along trajectories extending over an elongated beam path; collector means at the terminal end of the beam path for collecting and dissipating the energy of the collected electrons; electrical circuit means disposed along the beam path intermediate said gun means and said collector means in energy exchanging relation with the beam to produce output electromagnetic wave energy; means for magnetically focusing the beam over said beam path between said electron gun means and said collector means, said magnetic beam focusing means including, first and second pole piece structures disposed along said beam path in axially spaced relation relative to said beam path, each of said pole piece structures being made of a magnetically permeable material, magnet means for energizing said pole piece structures for producing an axially directed beam focus magnetic field composed of flux lines within the beam path, said first one of said pole piece structure having an aperture through which the beam passes, said aperture having a diameter such as to predominately determine the gradient of the axial field in the field region within one hole radius on either side of the transverse plane corresponding to an axial magnetic field intensity of 50 percent of the maximum axial magnetic field intensity between said pole pieces, said aperture diameter being so selected as to cause the flux lines of said magnetic field to conform to said electron trajectories within said region within one holE radius, said first pole piece structure also having a magnetic gap portion of lower magnetic permeability than that of the adjacent portion of said first pole piece structure, said gap being disposed surrounding said beam aperture in said first pole piece structure to allow an increase in the magnetic flux leakage through said first pole piece structure and being dimensioned so as to predominately determine the magnitude of the axial magnetic field intensity in the region of field external to said first and second pole pieces and in the region where the magnitude of the axial magnetic field is less than 10 percent of said maximum magnetic field intensity.
 2. The apparatus of claim 1 wherein said non-magnetically permeable gap is filled with a non-magnetic metal material.
 3. The apparatus of claim 1 wherein said electron gun includes, a thermionic cathode emitter having a concave cathode emitting surface facing said first pole piece structure, and a centrally apertured anode electrode spaced from said cathode for drawing a convergent flow of electrons from said cathode into said beam passing through said central aperture in said anode; said first pole piece structure being disposed facing said cathode and being shaped to produce a convergent bundle of magnetic flux lines passing through the peripheral area of the concave emitting surface of said cathode which substantially coincides with the convergent shape of the electrostatic field lines in the same region between said cathode and anode to produce a magnetically confined substantially laminar convergent flow of electrons from said cathode through said first pole piece structure.
 4. The apparatus of claim 3 wherein said gun is dimensioned to produce an area convergence of said beam in excess of
 50. 5. The apparatus of claim 1 wherein that portion of said first pole piece structure surrounded by said gap portion is connected electrically to said anode for operating at anode potential.
 6. The apparatus of claim 4 wherein said first pole piece structure includes an axial tubular projection of larger diameter than said beam aperture and being disposed surrounding said concave cathode emitting surface for shaping the beam focus magnetic field lines in the region of said concave cathode emitter.
 7. The apparatus of claim 6 wherein said first magnetic pole piece structure including the non-magnetic gap portion thereof is shaped to cause the magnetic field in the region of said electron gun to satisfy the relation AC BMAX Alpha /BC where AC is the area convergence of the electron beam; BMAX is the maximum value of axial magnetic field in the beam path intermediate said first and second pole piece structures; Alpha is the ratio of axial beam focus magnetic field in the beam path at the point of minimum beam diameter to BMAX; and BC is the value of axial magnetic field at the center of the concave cathode emitting surface.
 8. The apparatus of claim 1 wherein said magnet means for energizing said pole piece structures is a permanent magnet means. 